Field of the Invention
The present disclosure relates to non-destructive testing of materials, components, or structures for nonuniformities such as flaws, defects, and cracks. More particularly, embodiments of the present disclosure relate to methods and systems for non-destructive testing through measurement of magnetic flux leakage that is synchronous with induced changes in strain state in the material in the presence of an applied magnetic field.
Description of Related Art
Non-Destructive Testing (NDT) technologies, as the name implies, involves testing of materials for discontinuities and other defects in a way that does not damage the materials or otherwise impact their future usefulness. A number of conventional technologies can be used for NDT (also referred to as Nondestructive Examination (NDE), Nondestructive Inspection (NDI), and Nondestructive Evaluation (NDE)) including magnetic particle, liquid penetrant, radiography, ultrasonic testing, and eddy current, to name a few. Magnetic particle testing is useful for detecting surface and slightly subsurface discontinuities in ferromagnetic materials. Similarly, liquid penetrant testing can detect material defects open to the surface or to detect open surface cracks. However, neither of these technologies is capable of measuring subsurface flaws.
Eddy current testing is a more sophisticated method of detecting surface flaws. When an AC current flows in a coil in close proximity to a conducting surface the magnetic field of the coil will induce circulating (eddy) currents in that surface. The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance. This change in impedance can be measured, and used to determine the existence of a surface flaw. An Arrayed Eddy Current System is an adaptation of the basic eddy current measurement system. Placing a drive current and receiver coils on a flexible structure and rastering over a surface, a 2-D image of the surface can be determined. Certain companies have refined this approach to provide a very high probability of detection for surface flaws. The primary drawback is that this technology cannot directly measure the depth of the flaw.
Radiography and ultrasonic testing can be used to detect subsurface flaws. Radiography testing involves placing the material to be tested between a radiation source and a film or detector. Flaws in the material will result in differences in absorption recorded on the film or detector. Imaging techniques such as Computed Tomography (CT), Digital Radiography and Computed Radiography (CR) are replacing conventional film radiography in industrial radiographic testing. In ultrasonic testing, high frequency sound waves are transmitted through an object through a transducer, and cracks or other discontinuities will return reflected waves back to the transducer. However, defects identified by these methods may still be difficult to locate, or these methods may miss small defects entirely. Current NDE techniques are inadequate for detecting defects that are less than 100 um or that are located more than a few tens of microns below the surface. This is because such technologies use ultrasonic methods (good penetration—poor resolution) or eddy current methods (poor penetration—medium resolution) independently.
Emerging NDT technologies for detecting subsurface defects include thermal imaging, SQUID eddy current, and Synchrotron X-Ray diffraction. Thermal imaging can detect shallow subsurface defects, but it suffers from poor accuracy and resolution. SQUID Eddy Current techniques use an extremely sensitive magnetometer to detect subsurface flaws, but it needs to be kept at cryogenic temperatures, relegating it laboratory use only. Synchrotron X-Ray diffraction can be used to detect flaws through measurement of residual stress in a material, but this technique requires a particle accelerator and thus is not portable or usually practical. Thus, there is a need in the art for more sensitive, accurate, and practical NDT technologies capable of detecting both surface and subsurface defects.